Detectiou of early antigen pp65in blood leucocytes is a new method of diagnosing active cytomegalovirus (CMV) infection. Using a mixture of two monoclonal antibodies, CMV pp65 antigen was detected in blood leucocytes by a quantitative indirect immunofluorescence test within 3-5 hours after sampling. The pp65 antigen was detected in 20(29.8 %) of the 67 immunocompromised patients included in this study.